Acetylcysteine (N-acetylcysteine, NAC) is used in veterinary medicine as a mucolytic agent as an antidote for acetaminophen, xylitol, phenol toxicity and other hepatotoxic conditions in small animals.¹ In addition to its use as an antidote to acetaminophen activity, acetylcysteine is used for the treatment of canine degenerative myelopathy and as an adjunctive treatment of hepatic lipidosis in dogs and cats, to break up chondroids in the guttural pouch of horses and to break up meconium refractory neonatal foals. Its efficacy in the treatment of mares with Persistent Breeding-Induced Endometritis is also under study.²

Acetylcysteine is an acetylated derivative of the amino acid L-cysteine. Its mucolytic activity has made it useful in horses suffering from respiratory tract diseases involving increased mucus production.

Mucus Production in the Horse

The mucous membranes lining the horse’s nasal and pulmonary passages are sensitive. Materials (whether infectious or not) that aggravate them or causes inflammation can lead to production of additional mucus or fluid that is often evidenced by a runny nose and cough. 

In the case of a horse presenting with nasal discharge alone, the initial assessment is based on:

• Color
• Consistency 
• Amount
• Whether discharge is bilateral or unilateral
• Whether discharge has an odor and what type
• Whether blood is present

Horses with a bilateral watery or mucoid discharge may be reacting to environmental irritants or suffering from equine asthma or allergies.¹ Additional observations that may help to determine whether an allergic reaction or infection is or is not restricted to the nasal passages include:

• The horse’s demeanor
• Brightness/dullness of the eyes
• Exercise tolerance
• Whether the horse is eating and drinking normally
• Fecal and urine output
• Body temperature
• Presence of colic or abdominal discomfort.³

Acetylcysteine for Equines

While nasal discharge and couch in horses often results from environmental contaminants, among the most common bacterial causes of nasal discharge are Streptococcus equi zooepidemicus, which can lead to pneumonia, and S. equi equi,which causes strangles. Very often, horses with strangles do not develop nasal discharge until later in the disease process.³

In horses with strangles (Streptococcus equi subsp equi infection), acetylcysteine administered into the guttural pouch has been used to help break up chondroids and avoid the need for surgical removal. Acetylcysteine enemas have been used in neonatal foals to break up meconium refractory to repeated enemas. In mares in estrus, intrauterine acetylcysteine instillation could potentially be useful as an adjunctive treatment of endometritis, as acetylcysteine appears to have anti-inflammatory effects on equine endometrial cells and improves pregnancy rates.¹

Acetylcysteine is contraindicated (for pulmonary indications) in animals hypersensitive to it. Use as an antidote has no contraindications. Acetylcysteine administration via inhalation (nebulization) may cause bronchospasm, thus, it should be used very cautiously in animals with bronchospastic conditions (e.g., asthma, bronchitis).

When given orally for acetaminophen toxicity, acetylcysteine can cause GI effects (e.g., nausea, vomiting). IV administration appears to be well-tolerated in veterinary patients. Adverse effects reported when acetylcysteine is administered into the pulmonary tract include hypersensitivity, stomatitis, chest tightness, bronchoconstriction, and bronchial or tracheal irritation.¹

Where to buy Acetylcysteine

Acetylcysteine is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. ACETYLCYSTEINE 200 MG/ML INJECTABLE SOLUTION, 100ML by NexGen Pharmaceuticals is an excellent choice for the treatment of hepatotoxic conditions in small animals and as a mucolytic in horses.

Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

¹Plumb’s Veterinary Drugs.

²Caissie M.D., et. al., The Effects of Treatment with N-Acetyl Cysteine on Clinical Signs in Persistent Breeding-Induced Endometritis Susceptible Mares. J Equine Vet Sci. 2020 Sep;92:103142. doi: 10.1016/j.jevs.2020.103142. Epub 2020 May 30. PMID: 32797774.

³thehorse.com.